1. Field of the Invention
The present invention relates to a heating apparatus for heating a gas container filled with a liquified gas.
2. Description of the Prior Art
In the semiconductor-manufacturing industry, NH.sub.3, SiH.sub.2 Cl.sub.2, BCl.sub.3 and WF.sub.6 gases are used as a material gas. Each of these gases is usually stored as a liquified gas in a portable gas container which is called a gas cylinder or gas bomb. Such a gas is optionally supplied as an evaporated gas to gas consumers, as it evaporates in the gas container. However, unless sufficient heat corresponding to the heat of vaporization is supplied from the outside of a gas container to effect evaporation of the liquified gas, particularly when supplying gas from a gas container of liquified gas, the temperature of said liquified gas will drop over time as it is being used. If the temperature of a liquified gas drops, the pressure of its saturated vapor also lowers with the temperature drop. Thus, the supply of the liquified gas becomes ultimately impossible, because of a shortage of supply pressure.
Accordingly, the industry has adopted a method of using liquified gas while the flow rate of said gas, as supplied from one gas container, is limited. Alternatively, a heating apparatus is attached to the gas container so that a supplement of heat is provided to the liquified gas when it is desired to supply such gas over a limited flow rate. In general, the heating apparatus of the prior art is composed of a drum-shaped water jacket surrounding the side wall of a gas container and a warm water circulator for causing warm water, which is the heating source, to flow through this water jacket.
The aforedescribed conventional heating apparatus serves to transmit heat to the inside of a gas container by way of its side wall. In this method, however, there is a great waste of energy in transmitting heat not only to the liquified gas, but also to the gas existing above the liquified gas (the gas phase portion). When heating from the side, there is the additional problem that the thermal efficiency is low because the convection generated in the liquified gas is local.
Moreover, the warm water circulator used in the heating apparatus of the prior art is large in size, and hence a great deal of labor is needed for the handling, maintenance and management of the circulator.
It is therefore an object of the present invention to provide a heating apparatus for a gas container which avoids the aforedescribed problems of the prior art.